Binders based on hydrolyzates of silicic acid esters or silicic acid gels for solid particles are well known. Such binders are used in the paint and coating industries. Pigmentation of such binders, for example with zinc dust, results in paints (zinc dust paints) for anti-corrosion coatings on metals, especially on iron and steel. The cured coatings exhibit high surface hardness and thus excellent abrasion resistance, good solvent and heat resistance and electrical conductivity.
The first products in the development of zinc dust paints based on silicic acid gels were two-component systems where, in the simplest case, the first component was a hydrolyzate of ethyl silicate in alcoholic solution as a binder, and the other component was zinc dust. The two components had to be admixed immediately prior to application of the composition, because the coating composition solidified within a few hours after admixture of the zinc dust (U.S. Pat. No. 3,056,684). Subsequent developments utilized the effect of various additives on the processing properties and the quality of the paints, for example by addition of polyvinylbutyral (U.S. Pat. No. 3,392,130), boric acid esters (U.S. Pat. No. 3,392,036), alkyl titanates (U.S. Pat. Nos. 3,442,824 and 3,546,155) or trialkyl phosphates (German Patent 2 000 199). To increase the flash point of the binders, ethyl silicate has been transesterified with higher molecular alcohols (U.S. Pat. No. 3,730,743). Two-component zinc dust paints, however, have some disadvantages with respect to their use. The separate storage of binder and zinc dust causes storage space problems. Once the components are admixed, the mixture must be worked up immediately and completely within a few hours. In addition, ageing of the hydrolyzate during storage changes important technical parameters of use, such as processing properties (for example viscosity) and reactivity (effect on the curing rate), so that considerable problems can arise upon application of zinc dust paints comprising binders of different ages.
Because of these disadvantages, one-component systems were subsequently developed. They were specifically intended to improve the stability of the mixture of binders, pigments and fillers in the drum or pot. Because of the high density of zinc and the low viscosity of the alkyl silicate, the zinc dust settled at the bottom during storage in sealed containers so that it was difficult or impossible to resuspend the precipitate in the supernatant liquid by stirring. The use of specific rheologic additives such as BENTONE.RTM. or pyrogenic silica has made it possible to prepare one-component zinc dust paints which can be used without difficulties even after several months of storage. These formulations contain unhydrolyzed silicic acid esters and only react on contact with atmospheric moisture by hydrolysis and condensation to give up an increasingly crosslinked SiO.sub.2 gel (U.S. Pat. No. 3,859,101 and German Patent 26 54 036). The hydrolysis and condensation process is catalyzed with alkaline substances such as amines or alkoxides.
By virtue of the use of alcohols, ketones, ethers and alkyl esters of low molecular carboxylic acids as solvents or solvent mixtures the known binders often have a flash point which is lower than 21.degree. C. Since small amounts of alcohol are liberated by hydrolysis during use of zinc dust paints, it is difficult to achieve a flash point above 21.degree. C. with binders based on ethyl silicate. This is also true if solvents or solvent mixtures are used which originally have a flash point of significantly above 21.degree. C. When xylene is used as the solvent, the flash point of the binders is 23.degree. C. (European Patent 0 039 008). Likewise, the use of propyl or butyl silicates results in systems which have relatively high flash points. In both cases, however, the relatively slow evaporation of the solvent, be it xylene or the liberated alcohol, and the changed hydrolysis behavior of the esterified silicic acid groups delay the initiation of the hardening of the paints.